[Tutor] reading complex data types from text file

[Christian Witts]

Chris Castillo wrote:
> why does your 3rd and fourth lines have brackets?
>
> On Thu, Jul 16, 2009 at 1:08 AM, Christian Witts 
> <cwitts at compuscan.co.za <mailto:cwitts at compuscan.co.za>> wrote:
>
>     Chris Castillo wrote:
>
>         I'm having some trouble reading multiple data types from a
>         single text file.
>
>         say I had a file with names and numbers:
>
>         bob
>         100
>         sue
>         250
>         jim
>         300
>
>         I have a few problems. I know how to convert the lines into an
>         integer but I don't know how to iterate through all the lines
>         and just get the integers and store them or iterate through
>         the lines and just get the names and store them.
>
>         please help.
>         ------------------------------------------------------------------------
>
>         _______________________________________________
>         Tutor maillist  -  Tutor at python.org <mailto:Tutor at python.org>
>         http://mail.python.org/mailman/listinfo/tutor
>          
>
>     You could do it with a list comprehension
>
>     >>> names = []
>     >>> numbers = []
>     >>> [numbers.append(int(line.strip())) if line.strip().isdigit()
>     else names.append(line.strip()) for line in open('test.txt','rb')
>     if line.strip()]
>     [None, None, None, None, None, None]
>     >>> names, numbers
>     (['bob', 'sue', 'jim'], [100, 250, 300])
>
>     The list comprehension would unfold to
>
>     for line in open('test.txt', 'rb'):
>       if line.strip():
>           if line.strip().isdigit():
>               numbers.append(line.strip())
>           else:
>               names.append(line.strip())
>
>     And from there you can do what you like with the lists.
>
>     -- 
>     Kind Regards,
>     Christian Witts
>
>
>
 >>> [numbers.append(int(line.strip())) if line.strip().isdigit() else 
names.append(line.strip()) for line in open('test.txt','rb') if 
line.strip()]
[None, None, None, None, None, None]

Are you referring to these lines ?
If so, the reason is that for Python to recognize it as a list 
comprehension it needs to be wrapped in square brackets, if you were to 
use () instead to wrap around it it would become a generator expression 
(something which is incredibly powerful for larger amounts of data as it 
iterates when it needs to instead of pre-building everything.  And the 
following line with the Nones on is because that is the output of the 
calls to .append.  Normally you wouldn't see it in your application though.

-- 
Kind Regards,
Christian Witts